Generally, in an adhesive bonding portion between a metal member and a composite-material member, material characteristics, such as coefficient of thermal expansion and stiffness, change discontinuously, which causes a situation where stress concentration is likely to occur therein, resulting in debonding. Thus, the occurrence and development of the debonding between the metal member and the composite-material member lead to fracture of a structure including the adhesive bonding portion between the metal member and the composite-material member.
For example, a tank made of composite material (composite-material tank) is lightweight as compared to a tank made of metal (metal tank), so that it is expected to be used as an alternative to an existing metal tank, in the aerospace and automotive fields, etc. Under the current technologies, it is necessary to provide a metal mouthpiece in a connection portion with a pipe or the like. Thus, a tank cannot be entirely made of composite material, i.e., it inevitably has an adhesive bonding portion between a metal member and a composite-material member. Generally, in the adhesive bonding portion between the metal member and the composite-material member, material characteristics, such as coefficient of thermal expansion and stiffness, change discontinuously, which causes a situation where stress concentration is likely to occur therein, resulting in debonding, as mentioned above. The overall strength of the tank is borne by the composite material. Thus, the occurrence and development of the debonding between the metal member and the composite-material member lead to fracture of the tank.
Particularly, when the tank is used for storing cryogenic propellant (such as liquid hydrogen or liquid oxygen) in the aerospace field, it is placed in a situation where debonding is highly likely to occur and develop, because fracture toughness of an adhesive material is substantially decreased at cryogenic temperatures. With a view to preventing such debonding, a technique has been proposed in which, for an inner shell in which a peripheral portion around an opening of a tank is made of metal and an outer shell made of composite material, a fixing means is provided in the peripheral portion around the opening to constrain the inner shell and the outer shell with respect to each other (the following Patent Document 1).
Meanwhile, as for a gas reservoir tank to be used at normal temperature or an automobile fuel tank, there is no possibility of the debonding problem as long as a size and internal pressure thereof are maintained at current levels, because fracture toughness of an adhesive material is relatively high at normal temperature. However, differently from an index (strength) typically used in design, an index (energy release rate) governing the debonding increases in proportion to the second power of pressure (in the strength index, the first power of pressure) and increases in proportion to the first power of size (in the strength index, does not depend on size). Thus, if such a tank is increased in internal pressure and/or size in future, it is predicted that debonding between the metal member and the composite-material member also becomes a problem.
The bonding structure of the metal member and the composite-material member, with the adhesive bonding portion between the metal member and the composite-material member, is also used: in a satellite body, as a bonding structure of a thrust tube made of composite material (composite-material thrust tube), and a bracket made of metal (metal bracket), wherein the metal bracket is used to join the composite-material thrust tube and a top and/or bottom panel together; and, in a rocket, as a bonding structure of a skin plate made of composite material (composite-material skin plate) and formed to constitute a tail fin, and a fitting and joined to a rocket body, wherein the fitting is used to join the composite-material skin plate and the rocket body together. In these bonding structures, debonding between a metal member and a composite-material member also becomes a problem.